FIG. 5 is a schematic drawing of the lower portion of a conventional projection-exposure apparatus of a symmetric magnetic doublet type (SMD) configuration, that uses an electron beam and a segmented reticle (mask). In FIG. 5, the electron gun and illumination-optical system of the exposure apparatus are omitted so that only the configuration from a reticle 1 to a sensitive substrate (wafer) 5 is shown. This conventional exposure apparatus comprises a first projection lens 3 and a second projection lens 4.
By "segmented" is meant that the reticle is divided into multiple "mask subfields" that are individually exposed onto the wafer 5. The individual images of the mask subfields on the wafer, termed "transfer subfields," are positioned relative to each other such that the transfer subfields collectively "stitch together" the entire mask pattern on the wafer.
Each of the subfields 1a of the reticle 1 is uniformly illuminated by an electron beam EB having a cross-sectional profile that is similar in shape to the illuminated mask subfield 1a. The electron beam EB passing through the mask subfield 1a is focused on the wafer 5 through an aperture 6, whereby the resist in a corresponding transfer subfield on the wafer 5 is exposed to the electron beam EB. The aperture 6 is positioned at a crossover Z.sub.c on an optical axis AX. Off-axis portions EB1, EB2 of the electron beam EB pass through points 11, 12, respectively, on the mask subfield 1a. The off-axis electron-beam portions EB1, EB2 pass through the crossover Z.sub.c and converge on predetermined corresponding locations on the wafer 5.
In the FIG. 5 exposure apparatus, the projection lenses 3, 4 are sized and positioned so as to comprise an SMD lens system having a magnification of M (e.g., 1/2, 1/4, etc.). The size of the projection lens 3 is 1/M times the size of of the projection lens 4, and the positional relationship between the projection lens 3 and the projection lens 4 is 1:M with respect to the crossover Z.sub.c. In addition, the projection lenses 3, 4 each have the same AT (Ampere Turn) number, and are excited so that the directions of their respective magnetic fields formed on the optical axis AX are opposite to one another. Furthermore, the ratio of the distance between the reticle 1 and the aperture 6 to the distance between the aperture 6 and the wafer 5 is 1:M.
FIG. 6 is a graph qualitatively showing the distribution of the magnetic field along the optical axis that is induced by the projection lenses 3, 4. The vertical axis represents the strength of the magnetic field on the optical axis, and the horizontal axis represents a Z position along a Z-axis coordinate system which coincides with the optical axis AX. The Z-axis reference points Z.sub.0, Z.sub.i represent the Z-axis coordinates of the reticle 1 and the wafer 5, respectively. If the crossover Z.sub.c is selected as the origin of the Z-axis coordinate system (i.e., Z.sub.c =0), an SMD lens system generally satisfies the relation: EQU B.sub.1 (Z)=-M.multidot.B.sub.2 (-M.multidot.Z) (1)
wherein B.sub.1 (Z) is the on-axis magnetic-field distribution on the reticle side of the crossover Z.sub.c, and B.sub.2 (Z) is the on-axis magnetic-field distribution on the wafer side of the crossover Z.sub.c.
A characteristic feature of exposure apparatus that incorporate such an SMD lens system is that the chromatic aberration and distortion in both the magnification and the rotation caused by the projection lens 3 are canceled by the chromatic aberration and distortion in magnification and rotation caused by the projection lens 4, thereby minimizing the generation of these aberrations as a whole. While the SMD lens system is effective in minimizing the foregoing chromatic aberration and distortion, it does not effectively reduce deflection chromatic, deflection distortion, and deflection hybrid distortion aberrations. As a result, even if the pattern area on the reticle 1 is divided into multiple mask subfields 1a so that each mask subfield is individually exposed by the electron beam, the field curvature in mask subfields that are positioned away from the optical axis AX will be only slightly reduced by refocusing, resulting in a blurred and/or distorted image.